jmc

The given ratio tells us that $\frac{b}{a}=3$ or that $b=3a$. We substitute this value for $b$ so we have an equation with only one variable. We find $$\begin{array}{rl}3a& =12-5a\\ \Rightarrow 8a& =12\\ \Rightarrow a& =12/8\\ \Rightarrow a& =\boxed{\frac{3}{2}}.\end{array}$$